The present invention relates to granules based on pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol, to a method of producing them, and to their use.
It is known to prepare pyrogenic silicon dioxide (silica) from SiCl4 by means of high temperature or flame hydrolysis (Ullmanns Enzyklopxc3xa4die der technischen Chemie, 4th edition, Volume 21, page 464 (1982)).
Pyrogenic silicon dioxides are distinguished by the fact that they are extremely finely divided, have a high specific surface area (BET), very high purity, spherical particle form, and by the absence of pores. On account of those properties, pyrogenically prepared silicon dioxides are increasingly of interest as supports for catalysts (Dr. Koth et al., Chem. Ing. Techn. 52, 628 (1980). For that application, the pyrogenically prepared silicon dioxide is shaped by mechanical methods by means of, for example, tabletting machines.
From document EP 0 995 718 A1 there is known a pyrogenically prepared silicon dioxide in powder form doped with aluminum oxide by means of an aerosol. For the use of that silicon dioxide as a catalyst support, the object was to find a suitable granular form.
An object of the present invention was, therefore, to develop granules of pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol which can be used as a catalyst support.
The above and other objects of the present invention can be achieved with spray dried granules based on pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol, which granules have the following physico-chemical characteristic data:
In a preferred embodiment of the invention, the granules can have the following physico-chemical characteristic data:
The granules according to the invention can be produced by dispersing in water pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol, spray drying the dispersion, and optionally tempering the resulting granules at a temperature of from 150 to 1100xc2x0 C. for a period of from 1 to 8 hours.
The pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol may be a pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol in which the base component is a silicon dioxide that has been prepared pyrogenically in the manner of flame oxidation or, preferably, of flame hydrolysis and that is doped with a doping component of from 1.10xe2x88x924 and up to 20 wt. %, the doping amount preferably being in the range from 1 to 10,000 ppm and the doping component being a salt or a salt mixture of aluminum or a suspension of an aluminum compound or of metallic aluminum or mixtures thereof, the BET surface area of the doped oxide being from 5 to 600 m2/g, preferably in the range from 40 to 100 m2/g.
The silicon dioxide according to the invention can have a DBP number of less than 100 g/100 g.
The pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol can be prepared by feeding an aerosol into a flame such as is used for the pyrogenic preparation of silicon dioxide in the manner of flame oxidation or, preferably, of flame hydrolysis, mixing the aerosol homogeneously with the gas mixture of the flame oxidation or flame hydrolysis before the reaction, then allowing the aerosol/gas mixture to react to completion in the flame and separating the resulting pyrogenically prepared silicon dioxide doped with aluminum oxide from the gas stream in a known manner, there being used to produce the aerosol an aqueous solution containing salts or salt mixtures of aluminum or the metal itself in dissolved or suspended form or mixtures thereof, the aerosol being produced by atomization by means of a two-component nozzle or by a different method of aerosol production, preferably by means of an aerosol generator by ultrasonic atomisation.
There may be used as salts: AlCl3, Al2(SO4)3, Al(NO3)3.
The methods of flame hydrolysis for the preparation of pyrogenic oxides and also for the preparation of silicon dioxide (silica) are known from Ullmanns Enzyklopxc3xa4die der technischen Chemie, 4th edition, Volume 21, page 464.
The invention also provides granules based on pyrogenically prepared silicon dioxide doped with aluminum oxide by means of an aerosol, which granules have the following physico-chemical characteristic data: